This invention relates to particulate injection molding, and more specifically, to a method of controlling part density and dimensional accuracy of intricately shaped parts throughout the molding process.
I. Description of the Prior Art
It is desirable in many manufacturing processes to produce shaped parts having controlled porosity and density. These are then used as precursors for subsequent processing. The controlled porosity part may also be a final part itself, but usually production of this part is an intermediate step in the process. Or, the precursor is used to assist in manufacturing another article, which article is the objective of the process. By porosity control is meant that no porosity gradient exists from location to location within the part. Dimensional accuracy means the part's shape, as defined by dimensional metrology methods, is within specified dimensional tolerances. Usually, and especially when subsequent processing involves a heating step, porosity control and dimensional accuracy are related through the part's density. Porosity gradients and density gradients are related, and the two terms may be used nearly interchangeably. Porosity or density gradients are undesirable, especially if dimensional accuracy, part strength, gas permeability, heat sensitivity of the precursor, or other attributes are important during subsequent processing.
There are several known mechanisms by which precursors are produced. For example, large yet lightweight parts can be made of thermoplastic or thermosetting plastics having a solid exterior and a porous, foamed interior. The technology for producing such parts is a variant of plastic injection molding and is discussed in "New Methods Increase Design Freedom", Plastics World, Feb. 1991, Pg. 37. Generally these techniques involve the introduction of an immiscible gas, or a latently reactive solid, into a thermoplastic or thermosetting material. When injected into a mold cavity, a foam is created through the release of gas from a liquid species via a chemical or physical reaction. The thermosetting or thermoplastic material may also contain an amount of particulate material.
Another method for producing particulate bearing, porous shaped parts is to introduce into a thermoplastic, thermosetting, or a mixture of thermoplastic or thermosetting compounds, an amount of solid particulate material, the interstices of which eventually define the pore space of the precursor. The amount, shape, and size of the particulate is chosen so the mixture of particulates and plastics has sufficient fluid properties to be molded in a manner similar to molding the plastic without the particulate; i.e., by injection molding, extrusion, etc. Once the shape of the part is made using these techniques, the plastic residing in the interstices between particulate grains is removed through a variety of ways, for example, pyrolysis, solvent extraction, melt wicking, or combining with a reactive gas. The precursor thus is composed of a particulate material with open pores, and little or no plastic material.
Open pores, as used herein, means the pores of the part do not contain a species that would require an extra process step to remove. For example, if the pores contain water, they are considered open if subsequent processing of this component requires a heating step, and the water is removed without in any manner hindering the overall process.
Slip casting is another method. This involves mixing a particulate material (usually a fine, ceramic material) with a liquid material containing dispersants. This produces a dry, non-flowable material rendered flowable for a period of time during which it is flowed into a mold. The mold is then sufficiently porous to remove the liquid component, called the solvent, through its pore structure. Removal of the solvent causes the ceramic material to achieve a substantial green strength. The mold is subsequently removed from the part, leaving a porous part.
Cores and molds are made for use in castings by mixing sand as the particulate species with various binders The binders can be, for example, thermoplastic, thermosetting, organic oils, either foaming or non-foaming. But, unless they are of the foaming variety, the binder particulate mixture is not capable of being moved in a liquid manner and is usually blown or rammed into a mold. Once the mold is full, the binder is solidified using whatever means activates the particular binder in use, usually heat, but catalytic gasses or other catalytic additives are also used. Since the binder never occupied the entire pore space of the particulate to begin with, a porous precursor, the core or mold, is produced with the desired permeability and collapsibility for casting molten metal.
II. Disadvantages of the Prior Art
The relationship between precursor porosity and final dimensions arises when particulate preforms are heated in a process known as sintering to fuse the particles together. The relationship is based on three principles. (1) Since controlling porosity in a particulate filled part is the same as controlling the distribution of particulates from location to location in the part (the particulates having a mass), controlling porosity is equivalent to controlling density of the part. (2) Since the fusing together of the particulates in a porous part is accompanied by a decrease in porosity, and thus a decrease in volume of the part, less volume change occurs in high density locations compared to low density locations due to the low initial porosity of high density locations. (3) Since a change in volume of the part is easily and practically ascertained by the dimensional change or shrinkage of the part during sintering, non-uniform volume changes are manifested as non-uniform shrinkage. Therefore, non-uniform porosity yields non-uniform shrinkage. Non-uniform shrinkage will warp the part, or at least lead to out-of-tolerance dimensions of the final part.
For all the aforementioned processes, which utilize plastic materials to completely fill the space between the particulates, it will be understood that macro-scale average pore size can be controlled through material selection, mixing, or other parameters. Further, the distribution of pore sizes throughout the sections of a single part can only be effected by the external pressures acting on the fluid mass as it is flowed into and pressurized to fill the mold cavity. But pressures and stresses from within the material (or those, such as friction with the mold wall, that cannot be directly controlled) also exist. So does interaction of solidified and/or stratified layers of material with the mixture, this occurring at the mold/material interface. Also, there are varying solidification rates of the material due to variations in the time the material has been exposed to the mold, or the temperature differential in the mold environment. In addition, the externally applied pressure is used primarily as the external force moving the liquid material into the mold and filling the sections of the mold. As a result, its ability to achieve even density gradients is limited.
Improper control of any of the above listed factors sets a stress profile into the material when it solidifies or cures, and creates density gradients in the part when it is removed from the mold. This phenomenon is discussed in Gaspervich's article: "Practical Applications of Flow Analysis in Metal Injection Molding", The International Journal of Powder Metallurgy, Volume 27, No. 2, pp 133-139. Because of the potential of creating gradients, considerable effort is required when molding parts having tight dimensions to select with great accuracy the proper time sequences, filling pressures, temperatures, gate and runner arrangement and location, and other parameters to insure any resultant stress profile of the liquid mixture is as uniform throughout all the sections of the part as possible.
In addition to the density control problem, the precursor thus produced requires further processing in order to open the pores for use as precursors in other processes. Further, when dealing with mixtures of particulates, there is a tendency for segregation of the particulates from the liquid components of the mixture as the mix is filling the mold cavity. This is due to differences in flow characteristics between the solids and liquids, density differences, and the varying shear stresses and velocities encountered in the varying thickness of a mold cavity. For that matter, the particulate material itself will segregate according to resistance to flow, generally according to particle size.
Control of porosity and density are very important when the precursor is used for foundry cores and molds. It is desirous not to produce areas in the core and mold having excessively high porosity and low density, as this makes for a weak mold or core, and causes defects such as burn-in. Also, too high a density and too low a porosity is not desirable as gas permeability is consequently low and venting of molding gases is impaired, this leading to gas defects in the casting. A uniform flow of particulates is also required for uniform porosity. In foundry core processes, where particulates do not flow in a fluid manner, density gradients are inherent and deleterious to casting, and are caused by the inability of the particles to flow in a fluidized manner. Particles interlock, forming regions having a higher density than that occurring in the remainder of the piece.
Slip casting produces a porous article directly from the mold. But, slip casting is inherently slow and not capable of very precise or intricately detailed parts. In other processes for producing porous articles, strength, surface finish, permeability and other attributes are adversely affected by lack of control over the porosity and density of the part.